It's a universal belief that life is unfair, though there are many ways in which people manifest this belief. Some withdraw from the world, while others engage with it in an attempt to correct imbalances. Sometimes this engagement takes the form of anger.
The grumpy old man is a cliche that requires no explanation. In the Nov. 24 issue of the weekly magazine Shukan Asahi, a neurologist says the layer of the brain that moderates rational thought deteriorates as we age, and thus the filters we use to maintain decorum fail. It's why older people get angry more easily than younger people do, especially in public and with strangers. The article submits another, less physiological factor: Once a person retires they are less beholden to behavioral restrictions dictated by social norms.
The article tries to come to grips with a phenomenon called "buchigire rōjin," or "old people who fly off the handle," which seems to be a problem right now, judging by the amount of media attention it has received. Asahi gives several examples of the phenomenon, including an old man who loudly scolded a young woman on a train because he thought her daughters were "laughing too loud"; and older people waiting at a hospital pharmacy for their medical bill screaming at the clerk because they are being made to wait. The clerk, who, according to the terms of the social contract, is "lower" than the elderly people yelling at her, responds to the abuse with increasingly desperate apologies, which provokes their anger even more.